Plasmid - Research Article from Macmillan Science Library: Genetics

Plasmids are naturally occurring, stable genetic elements found in bacteria, fungi, and even in the mitochondria of some plants. They may be composed of DNA or RNA, double-stranded or single-stranded, linear or circular.

The transfer gene (tra) makes a protein that binds to the origin of replication site (ori). The protein nicks the DNA, relaxing it and allowing it to be transferred to another cell.

Plasmids almost always exist and replicate independently of the chromosome of the cell in which they are found.

Types of Plasmids

Plasmids are not usually required by their host cell for its survival. Instead, they carry genes that confer a selective advantage on their host, such as resistance to heavy metals or resistance to naturally made antibiotics carried by other organisms. Alternatively, they may produce antibiotics (toxins) that help the host to compete for food or space. For instance, antibiotic resistance genes produced...